The Mechanism of Water Evaporation

Walter Drisdell

Despite decades of study, water evaporation is still very poorly understood, with reported evaporation rates varying over three orders of magnitude. This seriously impedes atmospheric and geological modeling of isotope composition and cloud formation. With new experiments in the Saykally lab we hope to resolve this issue by focusing on the determination of the molecular mechanism for water evaporation.

By using liquid microjets with small aperture diameters (between 5 and 7 microns), we can inject liquid water into high vacuum such that re-condensation of evaporating molecules is negligible. This allows the study of pure evaporation without experimental complications due to condensation. By introducing a piezo-electric ceramic to the microjet assembly we have created a Vibrating Orifice Aerosol Generator (VOAG) which produces a stream of droplets of uniform size, allowing for very accurate modeling of the evaporation process to complement the experimental results.

Initial studies utilized mass spectrometry to examine the relative evaporation rates of different water isotopes. We have recently used Raman thermometry measurements to extract evaporation rates from the cooling of water droplets as a function of time. These experiments have shown that there is a small enthalpic or entropic barrier to water evaporation that limits the rate, in contradiction to the prevailing views.. Further experiments will utilize Cavity Enhanced Raman Spectroscopy (CERS) to augment the Raman thermometry measurements. This will allow the determinations of the size evolution of droplets with time, and will allow the temperature dependence of the barrier to be examined. Additionally, CERS measurements will help determine whether the barrier is primarily enthalpic or entropic.

Measurements are underway for H2O and D2O, and the effects of dissolved salts and surfactant molecules on the evaporation of both species will be examined. See publication 322.